But the others are again deferred but i can't connect to the remote servers with telnet (but ping, traceroute and DNS check are ok). The telnet connection on port 25 is it always possible when a remote server is ok ?

The servers that you can connect to (ISPConfig) are in your local network, and the others are outside your local network? Maybe it's a router problem. A customer of us once had the same problem until he found out it was caused by his router. What make is your router?